1. Field of the Invention
The present invention is in the field of jacketed centrifuges for material transfer between liquids, and includes a coaxial drum and conveyor worm assembly, with improved discharge means for separating the solids from the remaining liquids.
2. Description of the Prior Art
In jacketed centrifuge assemblies which customarily operate according to the counterflow principle, a relatively high specific gravity liquid containing suspended solids is brought into contact with a lighter specific gravity liquid which is suitable for use as an extraction agent. The contact takes place in a material exchange zone of the centrifuge drum. Prior to their discharge, the liquids are subjected to another clarification step. The centrifuge thus produces three fractions, the relatively light liquid loaded with extract, a relatively heavy extracted liquid, and a solids fraction.
Counterflow centrifuges of this type are shown in German AS 1,037,417. In this patent, there is disclosed a cylindrical drum into which a heavy liquid is introduced in proximity to the rotational axis, and is guided by means of channels having a spiral shape into countercurrent relation to a light liquid which is fed into the circumferential region of the drum under pressure. The material exchange process takes place in the spiral-shaped channels. At the outermost circumferential region, and in the region next to the axis of rotation, there are clarifying zones provided for the heavy and light liquids. Consequently, the solids together with the heavy liquid are removed by means of a channel which begins at the outermost circumferential region. The disadvantage of this type of centrifuge is that essentially only those solids can be discharged which are kept in suspension with the heavy liquid and can be removed as a suspension. It is not possible to continuously discharge solids which are heavier in specific density than the relatively heavy liquid.
There is a centrifuge described in German Pat. No. 2,701,763 wherein a heavy liquid containing solids is passed into the exchange zone of a drum which tapers conically in the direction toward both faces, the liquid being directed in countercurrent relation to a relatively light liquid. The heavy liquid is introduced in a region existing closer to the axis of rotation of the drum than the light liquid so that the latter must be fed in under pressure. On both sides of the material exchange zone there are located clarifying zones for the heavy and light liquid. The discharge of the solids proceeds with the assistance of a conveyor worm acting on the heavy liquid. In one form of the invention, there is a drying zone following the clarifying zone for the heavy liquid, such drying zone tapering conically in the direction of a solid discharge. The discharging of the heavy liquid takes place by means of a dip tube which projects into the drum. The disadvantage of this type of centrifuge is that the heavy liquid adhering to the solids flows in the drying zone oppositely to the conveying direction of the solids to the dip tube. Consequently, solids which have a weak, yeast-like consistency and have a density about that of the heavy liquid are taken up and discharged together with the heavy liquid. A mixture of such solids with the heavy liquid is particularly disadvantageous when the heavy liquid is subjected to a subsequent distillation in order to recover any extraction agent present. A further disadvantage is the fact that the light liquid must be fed into the drum under pressure. This requires an arrangement of pressure increasing elements in the conduit carrying the light liquid, resulting in an additional energy requirement.